The aeronomy experiments on the Pioneer Venus Orbiter (PVO) have established three characteristic features of the nightside ionosphere of Venus: (1) disappearing ionospheres, (2) large spatial/temporal variability of ion densities and (3) plasma holes/troughs. As the nightside ionosphere is essentially maintained by the transterminator flow of O + ions from the dayside, this flow should then be the major parameter controlling these features. Disappearing ionospheres, for example, have indeed been found to occur during episodes of high solar wind dynamic pressure (Psw), when the height of the terminator ionopause is greatly reduced and thus the transterminator flow severely diminished. In this paper, we study the other two features, viz the temporal/spatial ion density variability and the location of plasma holes/troughs by analyzing O + density profiles measured by the ion mass spectrometer experiment on the PVO. We find that most of the variability in the central nightside ionosphere is related to Psw, which is seen to control the O + peak density. Further, we observe that the altitude of the plasma holes/troughs is also dependant on Psw.
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